The II in 'GMT Master II' is joined at top and bottom so it forms a rectangle. As opposed to II being the stick dial. So unofficially it's a mk3 dial.
I actually like the rectangle dials but really where the money is on these is where it has a 3186 cal movement. The 3185 is not rare, and neither is a rectangular dial with a 3185 movement. In fact Rolex had been putting rectangular dials in as service replacement dials.
So rectangular dial with 3186 is where the £££'s is, otherwise its fairly run of the mill. I struggled to sell one I had previously here, rectangular dial with 3185.
Sorry this was a rushed answer.
Edit: I'm assuming the one you linked is a 3186, at that price.